Is this the stage you are talking about?
Because I lost a thread there mysteriously as well. I was trying to pull the mobs up the stairs (because the proc from the iron atronach transformation fell there, and I didn't want to waste it), and I went near the back wall, and suddenly I got the message that I lost a thread and the stage was reset. I thought it was because I had kited too far back (I heard from others that kiting mobs by going near walls had caused them to lose threads--probably there's some sort of anti-cheat code that is too sensitive even for normal kiting), but it might have been the stairs. My camera was turned away from there at that moment, so not sure if mobs had gone under them. I've had mobs going under stairs in other stages though, but it didn't cause the stage to reset not did I lose a thread, and I was able to kill them with aoes. It's a shame losing threads to bugs, especially when runs are so arduous and time-consuming.
Is this the stage you are talking about?
Because I lost a thread there mysteriously as well. I was trying to pull the mobs up the stairs (because the proc from the iron atronach transformation fell there, and I didn't want to waste it), and I went near the back wall, and suddenly I got the message that I lost a thread and the stage was reset. I thought it was because I had kited too far back (I heard from others that kiting mobs by going near walls had caused them to lose threads--probably there's some sort of anti-cheat code that is too sensitive even for normal kiting), but it might have been the stairs. My camera was turned away from there at that moment, so not sure if mobs had gone under them. I've had mobs going under stairs in other stages though, but it didn't cause the stage to reset not did I lose a thread, and I was able to kill them with aoes. It's a shame losing threads to bugs, especially when runs are so arduous and time-consuming.
That's the one, if you're up on the upper area they'll rush that spot to the right of the stairs in your image and go under the terrain.